Steam turbine designs consist of static nozzle segments that direct the steam flow into rotating buckets that are connected to a rotor. In steam turbines, the nozzle construction is typically called a diaphragm stage. Typical diaphragm stages are constructed using one of two methods. The first method is a “band/ring” method that uses an assembly comprised of a plurality of airfoils contained in inner and outer bands and then that banded airfoil assembly is welded into inner (web) and outer rings. The second method involves welding airfoils directly to inner and outer rings using a fillet weld at the interface. The second method is typically used for larger airfoils, where access for creating the weld is possible.
However, there are drawbacks to using these methods. One drawback is the inherent weld distortion of both the flow path and the steam path sidewalls. In this regard, current methods of steam turbine nozzle construction consist of high heat input welds using significant amounts of metal filler or deep electron beam welds. This material and heat input causes the flow path to distort and the airfoils often need to be adjusted after welding and stress relief. The result of the distortion is turbine efficiency losses in the steam turbine flow path.
Other methods using single nozzle construction into rings still have welds and mechanical interfaces that are difficult to model and analyze. They also are not as robust to stress level due to the weld interface and interfaces between the nozzles. Another method is to put “hooks” on the nozzle and slide each nozzle into a circumferential groove in the carrier. This method is also difficult and time consuming to analyze using finite element methods for stresses. Additionally, the frequency analysis is not as robust due to in determinant and changing boundary conditions between the nozzles and the carrier.
Thus, in general, current methods of constructing nozzle diaphragms are costly and time consuming in both engineering and manufacturing and all of the current methods consist of some type of weld or mechanical interface between nozzle and rings.
“Bling” design nozzles are currently used very little in steam turbine design. A bling is basically an entire nozzle flow path that is machined out of two half rings with no welding or assembly features. The bling has many valuable design qualities. First, blings have much lower stress levels because there are no weld joints or mechanical discontinuities in the load path. Second, the airfoil tolerances can be greatly improved over welded techniques. Third, they are easier to design and have more determinant frequency characteristics. In this regard, the 3D modeling and finite element analysis of the stress and frequencies is simpler, quicker and more robust due to the simplicity of the design.
An issue with current bling constructions is the interface between the carrier and the bling. In most diaphragm designs there are “crush pins” or small spacers to keep a tight tolerance between the diaphragm and the casing in the axial direction. The spacers act to keep the diaphragm loaded in the aft direction against the steam face. This helps assembly and also aids in the removal of the diaphragm after years of operation. In this regard, after years of operation, corrosion occurs on the surfaces and if the diaphragm to casing interface is tight on both axial faces, then it would be very difficult to get the diaphragm out as it would tend to lock into place due to the corrosion. Blings also “roll” or deflect downstream more than the slid in nozzle design. Many diaphragms use only the crush pins on the lower half (usually 3) and the upper half has a larger gap to the front face. This at times allows the diaphragm upper half to unseat off the back face and allow debris to get behind the face and cause a leakage path.